This invention relates to a novel class of quaternary ammonium salts which are useful as a cationic polymerization initiator.
A variety of cationic polymerization initiators are known including Friedel-Crafts catalysts such as aluminum chloride, boron trifluoride-ether complex, photo-degradable onium salts (S, Se, Te), dially iodonium salts and the like. These known initiators are generally not selective with respect to the reaction temperature. Therefore, epoxy resins containing these initiators begin to cure even at room temperature.
Japanese Laid Open Patent Application (Kokai) Nos. 37003/83 and 37004/83 disclose another type of cationic polymerization initiators. They are aliphatic or aromatic sulfonium salts capable of generating carbonium cations upon heating to an elevated temperature. Initiators of this type are known as "heat-latent cationic polymerization initiator". Cation-polymerizable resins such as epoxy resins containing the heat-latent initiator are, therefore, normally inactive but capable of curing at a temperature above the cleaving temperature of the initiator. This provides a heat-curable, one-component epoxy resin composition having a greater storage-stability and a longer pot life.
Japanese Laid Open Patent Application (Kokai) No. 96169/88 and U.S. Pat. No. 5,070,161, both assigned to the assignee of this application, disclose heat-latent cationic polymerization initiators having an N-benzylpyridinium structure.
Unfortunately, the prior art heat-latent initiators suffer from a problem that their decomposition products are malodorous or their cleaving temperature is relatively high. This problem makes them impractical to use in coating compositions where such malodor or high baking temperatures are not desirable.
Accordingly, a strong need exists for a heat-latent cationic polymerization initiator which may eliminate or ameliorate this problem.